Managing metadata and data for a logical volume in a distributed and declustered system

ABSTRACT

Methods, apparatus and computer program products for a distributed system include dividing logical volume data into data subsets, and defining at least one distributedly storage configuration for the logical volume. Metadata for the logical volume is written to a first set of first metadata tables, and the first set of first metadata tables is divided into metadata subsets having a one-to-one correspondence with the data subsets. The metadata subsets are distributed among the multiple digital information devices, and the metadata is copied from the first set of first metadata tables to a second set of corresponding second metadata tables in a one-to-one correspondence with the first metadata tables, and the second metadata tables are distributed among the multiple digital information devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.13/863,831, filed on Apr. 16, 2013, which is related to U.S. PatentApplications titled “Essential Metadata Replication”, “ParallelDestaging With Replicated Cache Pinning”, “Fine-Grained Control of DataPlacement”, “Backup Cache With Immediate Availability”, “Destaging CacheData Using a Distributed Freezer” and “Logical Region Allocation WithImmediate Availability” filed on even date with the present application,and which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to data storage, andspecifically to distributing metadata to one or more backup nodes of astorage system.

BACKGROUND

In computer storage systems (also referred to herein as storagesubsystems), disk partitioning and logical volume management are used tomanage physical storage devices such as hard disk drives. In diskpartitioning, a single storage device is divided into multiple logicalstorage units referred to as partitions, thereby treating one physicalstorage device as if it were multiple disks. Logical volume managementprovides a method of allocating space on mass-storage devices that ismore flexible than conventional partitioning schemes. In particular, avolume manager can concatenate, stripe together or otherwise combineregions (a region, also referred to herein as a partition, is a sequenceof bytes having a specific length, typically one megabyte) into largervirtual regions that administrators can re-size or move, potentiallywithout interrupting system use.

To manage a given volume, a partition table can be utilized to definethe relationship between a logical address of a logical volume andphysical regions (also known as blocks and partitions) on the physicalstorage device. Upon receiving a request to perform an input/output(I/O) operation at a specific logical address on the given volume, astorage system can utilize the partition table identify the physicallocation on a storage device that corresponds to the specific logicaladdress.

SUMMARY

There is provided, in accordance with an embodiment of the presentinvention a method, including arranging multiple storage devices andmultiple digital information devices having respective memories tocommunicate within a network, dividing data of a logical volume intodata subsets, defining, for the logical volume, at least one storageconfiguration for the data subsets distributed among the respectivestorage devices, writing metadata for the logical volume to a first setof first metadata tables, each of the first metadata tables configuredas a master metadata table, dividing the first set of first metadatatables into metadata subsets having a one-to-one correspondence with thedata subsets, distributedly storing the metadata subsets among themultiple digital information devices , the storage configuration of thedata subsets independent from the storing of the metadata subsets,copying the metadata from the first set of first metadata tables to asecond set of corresponding second metadata tables in a one-to-onecorrespondence with the first metadata tables, each of the secondmetadata tables configured as a backup metadata table, distributedlystoring the second metadata tables among the multiple digitalinformation devices so that a given first metadata table and thecorresponding second metadata table are stored to a different one of themultiple digital information devices, and so that the correspondingsecond metadata tables for the first metadata tables on a first of thedigital information devices are distributed among at least an additionaltwo of the digital information devices.

There is also provided, in accordance with an embodiment of the presentinvention an apparatus, including multiple storage devices and multipledigital information devices arranged on a network and having respectivememories, and a separate processor coupled to each of the respectivememories and configured to divide data of a logical volume into datasubsets, to define, for the logical volume, at least one storageconfiguration for the data subsets distributed among the respectivestorage devices, to write metadata for the logical volume to a first setof first metadata tables, each of the first metadata tables configuredas a master metadata table, to divide the first set of first metadatatables into metadata subsets having a one-to-one correspondence with thedata subsets, to distributedly store the metadata subsets among themultiple digital information devices, the storage configuration of thedata subsets independent from the storing of the metadata subsets, tocopy the metadata from the first set of first metadata tables to asecond set of corresponding second metadata tables in a one-to-onecorrespondence with the first metadata tables, each of the secondmetadata tables configured as a backup metadata table, and todistributedly store the second metadata tables among the multipledigital information devices so that a given first metadata table and thecorresponding second metadata table are stored to a different one of themultiple digital information devices, and so that the correspondingsecond metadata tables for the first metadata tables on a first of thedigital information devices are distributed among at least an additionaltwo of the digital information devices.

There is further provided, in accordance with an embodiment of thepresent invention a computer program product, the computer programproduct including a non-transitory computer readable storage mediumhaving computer readable program code embodied therewith, the computerreadable program code including computer readable program codeconfigured to arrange multiple storage devices and multiple digitalinformation devices having respective memories to communicate within anetwork, computer readable program code configured to divide data of alogical volume into data subsets, computer readable program codeconfigured to define, for the logical volume, at least one storageconfiguration for the data subsets distributed among the respectivestorage devices, computer readable program code configured to writemetadata for the logical volume to a first set of first metadata tables,each of the first metadata tables configured as a master metadata table,computer readable program code configured to divide the first set offirst metadata tables into metadata subsets having a one-to-onecorrespondence with the data subsets, computer readable program codeconfigured to distributedly store the metadata subsets among themultiple digital information devices , the storage configuration of thedata subsets independent from the storing of the metadata subsets,computer readable program code configured to copy the metadata from thefirst set of first metadata tables to a second set of correspondingsecond metadata tables in a one-to-one correspondence with the firstmetadata tables, each of the second metadata tables configured as abackup metadata table, computer readable program code configured todistributedly store the second metadata tables among the multipledigital information devices so that a given first metadata table and thecorresponding second metadata table are stored to a different one of themultiple digital information devices, and so that the correspondingsecond metadata tables for the first metadata tables on a first of thedigital information devices are distributed among at least an additionaltwo of the digital information devices, and computer readable programcode configured to update.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a block diagram that schematically illustrates a storagesystem, in accordance with an embodiment of the present invention;

FIG. 2 is a block diagram that schematically illustrates an example ofstorage management units stored on a storage system, in accordance withan embodiment of the present invention;

FIGS. 3A-3C, referred to collectively as FIG. 3, are block diagrams thatschematically show functional elements of a module of the storagesystem, in accordance with an embodiment of the present invention;

FIG. 4 is a flow diagram that schematically illustrates a method ofinitializing a logical volume, in accordance with an embodiment of thepresent invention; and

FIG. 5 is a flow diagram that schematically illustrates a method ofmanaging a logical volume, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

When managing logical volumes, digital information devices such asmodules of a clustered storage controller (described hereinbelow)typically implement a storage configuration when storing data on one ormore storage devices. For example, a logical volume may store data to aRedundant Array of Inexpensive Disks (RAID) 6 8+2 storage configuration,and metadata detailing the storage configuration (i.e., how the data isstored to ten storage devices in a RAID array) can be stored to apartition table.

In addition to data and metadata, a digital information device typicallyimplements a write cache configured to store cache data for the logicalvolume. For a given logical volume, the data, the metadata and the cachedata may be referred to herein as layers of the given logical volume.

Embodiments of the present invention provide methods and systems forstoring layers of a logical volume to a distributed system in order toprovide balance and scalability, as well as resiliency during errorrecovery. In embodiments described herein the layers comprise logicalvolume data, metadata and operational data. As described hereinbelowexamples of metadata include, but are not limited to, partition tabledata, transaction distribution data and disk distribution data. Examplesof operational data are described hereinbelow. While embodiments hereindescribe three layers of a logical volume (i.e., data, metadata andoperational), any number of layers is considered to be within the spiritand scope of the present invention.

In embodiments described herein, the distributed system may beimplemented on a clustered storage controller comprising multiplemodules, wherein each module comprises multiple storage devices. Theclustered storage controller can implement a distributed file system(also known as a clustered file system) which can be shared by beingsimultaneously mounted on the multiple modules. The ability of theclustered storage controller to distribute specific data (e.g., volumedata, volume metadata and volume cache data) among the storage devicesin the clustered storage controller is referred to herein as“distributedly storing” the specific data.

For example, volume data for a given logical volume can be distributedlystored by defining a

RAID storage configuration utilizing at least four storage devices(i.e., RAID 6 2+2) for each volume data stripe. Likewise, metadata thatdefines the RAID storage configuration for the given logical volume mayalso be distributedly stored to the storage devices in order to enhanceresiliency of the metadata, as described hereinbelow.

The distributed system may comprise storage devices and multiple digitalinformation devices (e.g., modules of a storage controller, as describedhereinbelow) having respective memories that are configured tocommunicate within a network. While the configuration describedhereinbelow comprises each module of a storage controller havingrespective storage devices, other configurations are considered to bewithin the spirit and scope of the present invention. For example, thestorage devices may be arranged in a disk enclosure or a “Just a Bunchof Disks” (JBOD) configuration that is coupled to the network.

In some embodiments, a different (and independent) resiliency scheme canbe implemented for each layer of a given logical volume. For example,data can be stored in a RAID 6 configuration that is arranged to recoverfrom two hardware failures, while metadata and cache data can be storedto a storage configuration that is arranged to recover from threehardware failures.

Examples of operational data layers that may be maintained usingembodiments of the present invention include, but are not limited to,input/output (I/O) intensity statistics, time-dependent workloadpatterns, quality of service requirements, data compressibility,presence and degree of data duplication, and other data which may beused to improve the effectiveness of the storage system's handling ofparticular elements of data. In some embodiments, if a master tablestoring operational data is lost due to a component failure, it istypically desirable to restore the master table so that the improvedoperations can be resumed, but the guarantee of restoration need not beas strong, and the speed of restoration need not be as fast as for thevolume's data. Thus, it may be useful to apply different protectionsystems, and sometimes a lower level of resilience, to these operationaldata layers.

The independent distribution of master and backup tables for differentoperational data layers, including metadata and operation data such ascache data, enables that upon a module failure, the actions ofreconfiguring a backup to be a new master and of creating new backupscan be performed separately on each layer, and these actions may selectdifferent modules for these roles on different layers for operationaldata corresponding to a specific data subset. The independence enablesthat different decisions can be made at each layer regarding the degreeof fault tolerance, the strictness of maintaining balance, thegranularity of distributing the layer, and the urgency of restoring thelayer after a failure. These separate decisions offer the benefit of animproved system design that makes more effective use of limitedresources.

Therefore, based on speed and resiliency requirements, embodiments ofthe present invention enable each layer of a logical volume to beindependently optimized using different combinations of parameters suchas redundancy level, space efficiency, and speed and/or complexity ofaccess and update.

FIG. 1 is a block diagram that schematically illustrates a dataprocessing storage subsystem 20, in accordance with a disclosedembodiment of the invention. The particular subsystem shown in FIG. 1 ispresented to facilitate an explanation of the invention. However, as theskilled artisan will appreciate, the invention can be practiced usingother computing environments, such as other storage subsystems withdiverse architectures and capabilities.

Storage subsystem 20 receives, from one or more host computers 22,input/output (I/O) requests, which are commands to read or write data atlogical addresses on logical volumes. Any number of host computers 22are coupled to storage subsystem 20 by any means known in the art, forexample, using a network. Herein, by way of example, host computers 22and storage subsystem 20 are assumed to be coupled by a Storage AreaNetwork (SAN) 26 incorporating data connections 24 and Host Bus Adapters(HBAs) 28. The logical addresses specify a range of data blocks within alogical volume, each block herein being assumed by way of example tocontain 512 bytes. For example, a 10 KB data record used in a dataprocessing application on a given host computer 22 would require 20blocks, which the given host computer might specify as being stored at alogical address comprising blocks 1,000 through 1,019 of a logicalvolume. Storage subsystem 20 may operate in, or as, a SAN system.

Storage subsystem 20 comprises a clustered storage controller 34 coupledbetween SAN 26 and a private network 46 using data connections 30 and44, respectively, and incorporating adapters 32 and 42, againrespectively. In some configurations, adapters 32 and 42 may comprisehost SAN adapters (HSAs). Clustered storage controller 34 implementsclusters of storage modules 36, each of which includes an interface 38(in communication between adapters 32 and 42), and a cache 40. Eachstorage module 36 is responsible for a number of storage devices 50 byway of a data connection 48 as shown.

While the configuration of storage subsystem 20 in FIG. 1 shows eachmodule 36 comprising an adapter 32 that is configured to communicatewith SAN 26, other configurations of the storage subsystem areconsidered to be within the spirit and scope of the present invention.For example, in an alternative configuration, adapter 32 is included ina subset of modules 36.

As described previously, each storage module 36 further comprises agiven cache 40. However, it will be appreciated that the number ofcaches 40 used in storage subsystem 20 and in conjunction with clusteredstorage controller 34 may be any convenient number. While all caches 40in storage subsystem 20 may operate in substantially the same manner andcomprise substantially similar elements, this is not a requirement. Eachof the caches 40 may be approximately equal in size and is assumed to becoupled, by way of example, in a one-to-one correspondence with a set ofphysical storage devices 50, which may comprise disks. In oneembodiment, physical storage devices may comprise such disks. Thoseskilled in the art will be able to adapt the description herein tocaches of different sizes.

While the configuration of storage subsystem 20 shown in Figure has thestorage subsystem storing data to physical storage devices 50, otherstorage apparatuses are considered to be within the spirit and scope ofthe present invention. For example, storage subsystem 20 may store datato one or more data clouds or storage virtualization devices (SVD).

Each set of storage devices 50 comprises multiple slow and/or fastaccess time mass storage devices, herein below assumed to be multiplehard disks. FIG. 1 shows caches 40 coupled to respective sets of storagedevices 50. In some configurations, the sets of storage devices 50comprise one or more hard disks, which can have different performancecharacteristics. In response to an I/O command, a given cache 40, by wayof example, may read or write data at addressable physical locations ofa given storage device 50. In the embodiment shown in FIG. 1, caches 40are able to exercise certain control functions over storage devices 50.These control functions may alternatively be realized by hardwaredevices such as disk controllers (not shown), which are linked to caches40.

Each storage module 36 is operative to monitor its state, including thestates of associated caches 40, and to transmit configurationinformation to other components of storage subsystem 20 for example,configuration changes that result in blocking intervals, or limit therate at which I/O requests for the sets of physical storage areaccepted.

Routing of commands and data from HBAs 28 to clustered storagecontroller 34 and to each cache 40 may be performed over a networkand/or a switch. Herein, by way of example, HBAs 28 may be coupled tostorage modules 36 by at least one switch (not shown) of SAN 26, whichcan be of any known type having a digital cross-connect function.Additionally or alternatively, HBAs 28 may be coupled to storage modules36.

In some embodiments, data having contiguous logical addresses can bedistributed among modules 36, and within the storage devices in each ofthe modules. Alternatively, the data can be distributed using otheralgorithms, e.g., byte or block interleaving. In general, this increasesbandwidth, for instance, by allowing a volume in a SAN or a file innetwork attached storage to be read from or written to more than onegiven storage device 50 at a time. However, this technique requirescoordination among the various storage devices, and in practice mayrequire complex provisions for any failure of the storage devices, and astrategy for dealing with error checking information, e.g., a techniquefor storing parity information relating to distributed data. Indeed,when logical unit partitions are distributed in sufficiently smallgranularity, data associated with a single logical unit may span all ofthe storage devices 50.

While such hardware is not explicitly shown for purposes of illustrativesimplicity, clustered storage controller 34 may be adapted forimplementation in conjunction with certain hardware, such as a rackmount system, a midplane, and/or a backplane. Indeed, private network 46in one embodiment may be implemented using a backplane. Additionalhardware such as the aforementioned switches, processors, controllers,memory devices, and the like may also be incorporated into clusteredstorage controller 34 and elsewhere within storage subsystem 20, againas the skilled artisan will appreciate. Further, a variety of softwarecomponents, operating systems, firmware, and the like may be integratedinto one storage subsystem 20.

Storage devices 50 may comprise a combination of high capacity hard diskdrives and solid state disk drives. In some embodiments each of storagedevices 50 may comprise a logical storage device. In storage systemsimplementing the Small Computer System Interface (SCSI) protocol, thelogical storage devices may be referred to as logical units, or LUNs.While each LUN can be addressed as a single logical unit, the LUN maycomprise a combination of high capacity hard disk drives and/or solidstate disk drives.

FIG. 2 is a block diagram that schematically illustrates an example ofstorage management units (SMU) configured as slices 60 stored on storagedevices 50 of clustered storage controller 34 (also referred to hereinas a storage system), in accordance with an embodiment of the presentinvention. While the embodiments herein describe distributing metadatastorage for slices 60, distributing metadata storage for other types ofstorage management units is considered to be within the spirit and scopeof the present invention. For example, the embodiments described hereincan be used to distribute metadata for other types of storage managementunits such as logical volumes and storage pools.

Additionally, in the embodiments described herein, each module 36 may bereferenced by an identifier (A), where A is an integer representing agiven module 36. As shown in FIG. 2, there are four modules 36 that maybe referenced as module 36(1), module 36(2), module 36(3) and module36(4).

Furthermore, each storage device 50 may be referenced by an ordered pair(A,B), where A is defined above, and where B is a number representing agiven storage device 50 coupled to the given module via data connection48. For example, storage devices 50(3,1), 50(3,2), 50(3,3) and 50(3,4)are coupled to module 36(3) via data connection 48.

FIG. 3A is a block diagram that schematically illustrates functionalelements of module 36, in accordance with an embodiment of the presentinvention. Module 36 comprises a processor 62 and a memory 64. For agiven module 36 configured to include adapter 32, memory 64 comprises aninterface node 66 (i.e., not all memories 36 in storage system 20include the interface node). Memory 36 also comprises a transaction node68 and a distribution table 70. In operation, processor 62 executesinterface node 66 and transaction node 68 from memory 64.

Processor 62 typically comprises a general-purpose computer, which isprogrammed in software to carry out the functions described herein. Thesoftware may be downloaded to processor 62 in electronic form, over anetwork, for example, or it may be provided on non-transitory tangiblemedia, such as optical, magnetic or electronic memory media.Alternatively, some or all of the functions of processor 62 may becarried out by dedicated or programmable digital hardware components, orusing a combination of hardware and software elements.

Interface node 66 comprises a software application that is configured toreceive I/O requests from a given host computer 22, and to convey theI/O request to a given transaction node 68. Additionally, upon the giventransaction node completing the I/O request, interface node 66 conveys aresult of the I/O request to the given host computer. For example, ifthe I/O request comprises a write operation, then the conveyed resultmay comprise an acknowledgement of the write. Alternatively, if the I/Orequest comprises a read operation, then the conveyed result maycomprise data retrieved from storage devices 50.

Transaction node 68 comprises a software application that processes I/Orequests via multiple schedulers 72, which manage a set of slices 60.While the configuration of transaction node 68 shown in FIG. 3Acomprises four schedulers 72, any number of schedulers is considered tobe within the spirit and scope of the present invention. In someembodiments, processor 62 may execute each scheduler 72 on a separatethread (also known as a logical core) of the processor.

In embodiments described herein, each scheduler 72 may be referenced byan ordered pair (A,C), where A is defined above, and C is a numberrepresenting a given scheduler 72 executing within the given module. Inthe example shown in FIG. 3A, the first scheduler 72 in module 36(2) maybe referred to herein as scheduler 72(2,1), the second scheduler 72 inmodule 36(2) may be referred to herein as scheduler 72(2,2), the thirdscheduler 72 in module 36(2) may be referred to herein as scheduler72(2,3), and the fourth scheduler 72 in module 36(2) may be referred toherein as scheduler 72(2,4).

As described supra, storage controller 34 may configure a logical volumeas a set of slices 60, wherein each of the slices comprises a set ofregions on a given storage device 50. For example, a given logicalvolume may comprise four slices 60 spread over storage devices 50(1,2),50(2,3), 50(3,4) and 50(4,2). There may be instances where a givenstorage device 50 stores more than one slice for a given logical volume.Additionally, as described in detail hereinbelow, processor 62 may storemultiple copies of a given slice. For example, processor 62 may store afirst copy of a given slice 60 on a first storage device 50 (alsoreferred to herein as the primary storage device for the given slice),and an additional copy of the given slice on a second storage device 50(also referred to herein as the secondary storage device for the givenslice).

In embodiments of the present invention, each slice 60 can be associatedwith a first scheduler 72 that can be configured as a master scheduler,and one or more additional schedulers 72 that can be configured asbackup schedulers. Differences between the master and the backupschedulers are described hereinbelow. In the event of a failure of themaster scheduler, processor 62 can reconfigure one of the backupschedulers to function as the master scheduler, thereby ensuring thecontinuous availability of data stored in storage controller 34.

As described supra, processor 62 may store a first copy of a given slice60 on a primary storage device 50, and an additional copy of the givenslice on one or more secondary storage devices 50. In the event of afailure of the primary storage device, processor 62 can reconfigure oneof the secondary storage devices to function as the primary storagedevice, thereby ensuring the continuous availability of data stored instorage controller 34.

Processor 62 can store associations between the slices, the schedulersand the storage devices to distribution table 70. Distribution table 70comprises transaction distribution data 74 and disk distribution data76. Transaction distribution data 74 can be configured to storeassociations between the slices and the schedulers, and diskdistribution data 76 can be configured to store associations between theslices and the storage devices.

FIG. 3B is block diagram that schematically shows example entries intransaction distribution data 74 and disk distribution data 76, inaccordance with an embodiment of the present invention. In the exampleshown in FIG. 3B, each slice 60 is associated with a master scheduler 72and two backup schedulers 72, and a primary and a secondary storagedevice 50.

In the embodiments described herein, each slice may be referenced by anidentifier (D), where D is a number representing a given slice 60. Inthe configuration shown in FIG. 3B-3C, storage controller 34 comprises160 slices 60 that can be referenced as slice 60(1)—slice 60(160).Identifier D is also referred to herein as a slice number, so that eachslice 60 has an associated slice number, and in the example D is aninteger between 1 and 160.

As shown in transaction distribution data 74, scheduler 72(2,3) isconfigured as the master scheduler and schedulers 72(3,2) and 72(1,4)are configured as the backup schedulers (i.e., BACKUP-A and BACKUP-B asshown the figure) for slice 160(1). Additionally, as shown in diskdistribution data 76 for slice 60(1), storage device 50(1,1) isconfigured as a primary storage device and storage device 50(2,1) isconfigured as a secondary storage device.

While the configuration of disk distribution data in FIG. 3C showsslices 60 stored Redundant

Array of Inexpensive Disks (RAID) 10 configuration (i.e., each slice 60is mirrored once among multiple storage devices 50), other storageconfigurations are considered to be within the spirit and scope of thepresent invention. For example, slices 60 may be stored in a RAID 6(e.g., a RAID 6 6+2 or a RAID 6 8+2) configuration.

As shown in the Figures, for a given slice 160, the master scheduler,the backup scheduler(s), the primary storage device and the secondarystorage device(s) can be distributed among different modules 36 ofstorage system 20. Additionally, each module 36 may store any number(including zero) of master and backup schedulers 72.

FIG. 3C is a block diagram that schematically illustrates schedulers72(1,4), 72(2,3) and 72(3,2) in accordance with an embodiment of thepresent invention. Each scheduler 72 comprises pairs of partition tables78 and caches 40, wherein each of the pairs is associated with a givenslice 60. Each entry in a given partition table 78 corresponds to apartition (i.e., a region) on a given storage device 50, and comprises adata structure (e.g., an array) that enables processor 62 to map a givenvolume number and logical address to the partition. Operation of caches40 is described in FIG. 1, hereinabove.

As described supra, each scheduler 72 can be associated with a givenslice 60 and can function as either a master scheduler or a backupscheduler for the given slice. In the example shown in FIGS. 3B-3C, eachslice 60 has a master scheduler 72 (“MASTER”) and two backup schedulers72 (“BACKUP-A” and “BACKUP-B”). Likewise, each partition table 78 may bereferenced by an ordered pair (D,E), where D is a number representing agiven slice 60, and E describes a role of a given partition table 78,and each cache 40 may be referenced by an ordered pair (D,F), where D isdefined above, and F describes a role of a given cache 40. Inembodiments described herein each slice 60 has a master cache 40 and twobackup caches 40 (i.e., BACKUP-A and BACKUP-B).

Continuing the example described supra, the schedulers shown in FIG. 3Ccomprise the schedulers associated with slice 60(1). As shown in theFigure, scheduler 72(2,3) comprises partition table 78(1, MASTER) andcache 40(1, MASTER), scheduler 72(3,2) comprises partition table 78(1,BACKUP-A) and cache 40(1, BACKUP-A), and scheduler 72(1,4) comprisespartition table 78(1, BACKUP-B) and cache 40(1, BACKUP-B).

In embodiments described herein, processor 62 can map an I/O request toa given scheduler 72, as opposed to mapping the I/O request to a givenmodule 36 or a given storage device 50. By mapping I/O requests toschedulers 72, embodiments of the present convention “decouple”partition tables 78 from storage devices 50. In other words, uponreceiving an I/O request, interface node 66 may convey the I/O requestto a first module 36 executing a given scheduler 72 comprising a givenpartition table 78, wherein the given partition table references a givenstorage device 50 coupled to a second module 36.

In operation, a given partition table 78 can store metadata that detailsa storage configuration for the slice associated with the givenpartition table. Examples of metadata for each partition in a givenpartition table 78 include, but are not limited to:

-   -   A volume number of a given slice 60 associated with the        partition.    -   A starting logical address for the partition.    -   A location (i.e., an in-disk address, e.g., a track number) on a        given storage device 50 for the partition.    -   Timestamps indicating the last time the partition was accessed        and/or updated.    -   One or more flags.    -   A hash table entry. Partition table 78 may include a hash table        that enables processor 62 to rapidly translate a given volume        number and logical address to an entry in the partition table.    -   Pointers to next and previous partitions in a given volume.

In addition to the partition tables, metadata may also refer totransaction distribution data 74 and disk distribution data 76 thatprocessor 62 can replicate among modules 36.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system”.Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Python, Java, Smalltalk, C++ or the like and conventionalprocedural programming languages, such as the “C” programming languageor similar programming languages. The program code may execute entirelyon the user's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks. These computerprogram instructions may also be stored in a computer readable mediumthat can direct a computer, other programmable data processingapparatus, or other devices to function in a particular manner, suchthat the instructions stored in the computer readable medium produce anarticle of manufacture including instructions which implement thefunction/act specified in the flowchart and/or block diagram block orblocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

Distributed Data and Metadata Management

FIG. 4 is a flow diagram that schematically illustrates a method ofinitializing a logical volume whose data is stored on storage devices50, in accordance with an embodiment of the present invention. Inembodiments described herein, processor 62 divides data of the logicalvolume into data subsets (e.g., slices 60), and managing the logicalvolume comprises managing one or more storage configurations, metadataand caches 40 for the data subsets. In the example described in the flowdiagram, processor 62 stores the metadata to metadata tables such aspartition tables 78.

In an initial step 80, processor 62 defines metadata for at least onestorage configuration (e.g., a

RAID configuration) for a logical volume whose data is distributed amongstorage devices 50. For example, processor 62 may store the logicalvolume as multiple slices 60 on storage devices 50, and the slices maybe configured to store data in more than one RAID configurations.Maintaining multiple RAID configurations for different slices 60 of agiven logical volume is described in more detail in U.S. PatentApplication “Fine-Grained Control of Data Placement”, referenced above.

As described supra, processor 62 can store a logical volume's data tomultiple slices 60, and define a metadata table (e.g., a given partitiontable 78) and a given cache 40 for each of the slices. In a first createstep 82, processor 62 creates a first set of multiple first metadatatables, and configures each of the first metadata tables as a mastermetadata table.

In a first write step 84, processor 62 writes the defined metadata tothe first set of first metadata tables, and divides the first set of thefirst metadata tables into metadata subsets having a one-to-onecorrespondence with the data subsets. Each of the first metadata tablesmay be referred to herein as a master metadata table. Likewise, thefirst set of first metadata tables may be referred to as a set of mastermetadata tables.

In a second create step 86, processor creates a second set of secondmetadata tables, and configures each of the second metadata tables as abackup metadata table, so that the second metadata tables have aone-to-one correspondence with the first metadata tables. Each of thesecond metadata tables may be referred to herein as a backup metadatatable. Likewise, the second set of second metadata tables may bereferred to as a set of backup metadata tables. In a copy step 88,processor 62 copies metadata from the set of master metadata tables tothe set of backup metadata tables.

In addition to metadata, processor 62 may maintain an additional layerof operational data for a given logical volume. In operation operationaldata tables are configured to store operational data. In embodimentsdescribed herein the operational data tables comprises caches 40, andthe operational data comprises cache data.

In a cache create step 90, processor 62 creates a first set of firstcaches 40, divides the first set of the first cache tables into cachesubsets having a one-to-one correspondence with the data subsets, andconfigures each of the first caches as a master cache, thereby creatinga set of master caches. Processor 62 also creates at least one secondset of second caches 40 and configures each of the second caches as abackup cache, thereby creating a set of backup caches. In someembodiments, the backup caches have a one-to-one correspondence with themaster caches.

Finally, in a distribution step 92, processor 62 distributedly stores,among the respective memories of modules 36, the set of master metadatatables, the set of backup metadata tables, the set of master caches 40and the set of backup caches 40. When distributing the metadata tables,processor 62 can balance the metadata table distribution among modules36 so that a given master metadata table and its corresponding backupmetadata table are stored on different modules 36, and so that thecorresponding second metadata tables for the first metadata tables on afirst module 36 are distributed among at least two additional modules36.

In embodiments of the present invention, the storage configuration ofthe data subsets can be independent of the distributed storageconfiguration of the master and the backup metadata tables and themaster and the backup caches. Additionally the distributed storageconfiguration of the master and the backup metadata tables may beindependent of the distributed storage of the master and the backupcaches. For example, processor 62 can configure system 20 so that agiven one of the data subsets does not share a failure domain with thecorresponding master and backup metadata tables and the correspondingmaster and backup caches, as described in the configuration shown inFIG. 3.

Likewise, when distributing the caches, processor 62 can balance thecache distribution among modules 36 so that a given master cache 40 andits corresponding backup cache 40 are stored on different modules 36,and so that the corresponding second caches for the first caches on afirst module 36 are distributed among at least two additional modules36.

In some embodiments, processor 62 can distributedly store the secondmetadata tables so that the corresponding second metadata tables for thefirst metadata tables in each of the metadata subsets are distributedamong at least three of the digital information devices. Likewise,processor 62 can distributedly store the second caches so that thecorresponding second caches for the first caches in each of the cachesubsets are distributed among at least three of the digital informationdevices.

In additional embodiments, the metadata can be divided into the multiplemetadata tables so that a typical storage operation (e.g., a read or awrite operation) can be performed referencing no more than a smallnumber of the separate metadata tables and caches. For example, read andwrite operations may reference a single metadata table and a singlecache. As a result, when operations are distributed among the modulesaccording to the residence of the master metadata table, the work ofmaintaining the metadata can be spread among the modules.

Once the logical volume is initialized using the steps described in FIG.4, processor 62 can process storage commands that modify a given mastermetadata table and/or a given cache 40. For example, while processing arequest to write data to a logical volume, processor 62 can store thedata to a given master cache 40, and modify a timestamp in a givenmaster metadata table.

FIG. 5 is a flow diagram that illustrates a method of managing thelogical volume initialized in the steps described in FIG. 4. In a firstcomparison step 100, if the metadata in a given master metadata table ismodified (e.g., the processor modifies the metadata while processing astorage operation), then in a first update step 102, the processorupdates the corresponding backup metadata table with the modifiedmetadata.

In a second comparison step 104, if cache data in a given master cache40 is modified, then in a second update step 106, the processor updatesthe corresponding backup cache 40 with the modified cache data.Returning to step 100, if the metadata in the given master metadatatable was not updated, then the method continues with step 104.

In a third comparison step 108, if a given processor 62 in a firstmodule 36 detects a hardware and/or a software failure in a second givenmodule 36, then in a first reconfiguration step 110, the given processorreconfigures, for each master metadata table that was stored on thefailed module, the corresponding backup metadata table as a new mastermetadata table. In embodiments where there is more than one backupmetadata table for each master metadata table, the given processor canreconfigure, for each master metadata table that was stored on thefailed module, one of the corresponding backup metadata tables as thenew master metadata table.

In some embodiments, storage controller 34 may be configured to maintaina specific number of backup metadata tables for each master metadatatables. Therefore, in a first creation step 112, the given processorcreates a new second metadata table for each of the new master metadatatables, configures the new second metadata tables as backup metadatatables, and in a first copy step 114, the given processor copies themetadata from each of the new master metadata tables to thecorresponding new backup metadata tables.

In a second reconfiguration step 116, the given processor reconfigures,for each master metadata table that was stored on the failed module, thecorresponding backup cache 40 as a new master cache. In embodimentswhere there is more than one backup caches for each master cache, thegiven processor reconfigures, for each master cache 40 that was storedon the failed module, one of the corresponding backup caches 40 as thenew master cache.

In some embodiments, storage controller 34 may be configured to maintaina specific number of backup caches 40 for each master cache 40.Therefore, in a second creation step 118, the given processor creates anew second cache 40 for each of the new master caches, configures thenew second caches as backup caches, and in a second copy step 120, thegiven processor copies the cache data from each of the new master cachesto the corresponding new backup cache.

Finally, in a distribution step 122, the given processor distributedlystores, among modules 36, the new backup metadata tables and the newbackup caches, and the method returns to step 100. When distributing thenew backup metadata tables, the given processor can balance the metadatatable distribution among modules 36 so that a given master metadatatable and its corresponding backup metadata table are stored ondifferent modules 36. Likewise, when distributing the backup caches, thegiven processor can balance the cache table distribution among modules36 so that a given master cache 40 and its corresponding backup cacheare stored on different modules 36. Returning to step 108, if the givenprocessor 62 does not detect any hardware and/or a software failure inmodules 36, then the method returns to step 100.

As described in step 92 of the flow diagram shown in FIG. 4, processor62 may distribute both the second metadata tables corresponding to firstmetadata tables in each metadata set and the second caches correspondingto first caches in each cache set among multiple modules 36. In theevent of a failure (i.e., step 108), this distribution enables storagecontroller 24 to distribute the reconfiguration tasks (i.e., steps110-120) among multiple modules 36, thereby “balancing” thereconfiguration.

In some embodiments, processor 62 may ensure that the backup metadatatables and the backup caches which are reconfigured to become mastermetadata tables and master caches are not concentrated on a few modules36, which may, as a consequence, become overloaded. Therefore, whenaddressing the set of all the master metadata tables and the mastercaches whose master copy resides on any given module 36, it can beenhance resiliency if the backup responsibilities for those metadatatables and caches are distributed in a balanced way among the othermodules.

While the configuration described in the flow comprises a single backupfor each metadata table and cache 40, other configurations areconsidered to be within the spirit and scope of the present invention.For example, in the example shown in FIG. 3, processor 62 may maintaintwo backups for each master metadata table and each cache 40, anddistributes the backup metadata tables and the backup caches amongmodules 36 in order to enable storage controller 34 to recover from afailure of any two of the modules.

Additionally or alternatively, while the example described in thefigures implements redundancy and resiliency by storing, in a masterscheduler 72 for a given slice 60, a given master metadata table and agiven master cache 40, and storing, to a backup scheduler 72 for thegiven slice, a given backup metadata table and a given backup cache 40,other methods of implementing redundancy and resiliency for a givenslice is considered to be within the spirit and scope of the presentinvention. For example, processor 62 can distribute a first number ofbackup metadata tables among modules 36, and distribute a second number(different than the first number) of backup caches among the modules.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. A method, comprising: arranging multiple storage devices and multipledigital information devices having respective memories to communicatewithin a network; dividing data of a logical volume into data subsets;defining, for the logical volume, at least one storage configuration forthe data subsets distributed among the respective storage devices;writing metadata for the logical volume to a first set of first metadatatables, each of the first metadata tables configured as a mastermetadata table; dividing the first set of first metadata tables intometadata subsets having a one-to-one correspondence with the datasubsets; distributedly storing the metadata subsets among the multipledigital information devices, the storage configuration of the datasubsets independent from the storing of the metadata subsets; copyingthe metadata from the first set of first metadata tables to a second setof corresponding second metadata tables in a one-to-one correspondencewith the first metadata tables, each of the second metadata tablesconfigured as a backup metadata table; and distributedly storing thesecond metadata tables among the multiple digital information devices sothat a given first metadata table and the corresponding second metadatatable are stored to a different one of the multiple digital informationdevices, and so that the corresponding second metadata tables for thefirst metadata tables on a first of the digital information devices aredistributed among at least an additional two of the digital informationdevices.
 2. The method according to claim 1, and comprising uponmodifying the metadata in a one of the first metadata tables whileprocessing a storage request, updating the corresponding second metadatatable with the updated metadata.
 3. The method according to claim 1, andcomprising distributedly storing the second metadata tables so that thecorresponding second metadata tables for the first metadata tables onthe first of the digital information devices are distributed among atleast an additional three of the digital information devices.
 4. Themethod according to claim 1, and comprising upon detecting a failure ofa given one of the digital information devices storing one of the firstmetadata tables, configuring the corresponding second metadata table asa new master metadata table, creating a new second metadata table, andconfiguring the new second metadata table as a backup metadata table forthe new master metadata table.
 5. The method according to claim 1, andcomprising writing operational data for the logical volume to a firstset of first operational data tables having a one-to-one correspondencewith the data subsets, each of the first operational data tablesconfigured as a master operational data table, dividing the first set offirst operational data tables into operational data subsets, the storageconfiguration of the data subsets independent from the storing of theoperational data subsets, distributedly storing the operational datasubsets among the multiple digital information devices, copying theoperational data to a second set of corresponding second operationaldata tables in a one-to-one correspondence with the first operationaldata tables, each of the second operational data tables configured as abackup operational data table, distributedly storing the first and thesecond operational data tables among the multiple digital informationdevices so that a given first operational data table and thecorresponding second operational data table are stored to a differentone of the multiple digital information devices, and so that thecorresponding second operational data tables for the first metadatatables on a first of the digital information devices are distributedamong at least an additional two of the digital information devices, andupon modifying the operational data in a one of the first operationaldata tables while processing a storage request, updating thecorresponding operational data table with the updated operational data.6. The method according to claim 5, and comprising distributedly storingthe second operational data tables so that the corresponding secondoperational data tables for the first operational data tables on thefirst of the digital information are distributed among at least anadditional three of the digital information devices.
 7. The methodaccording to claim 5, and comprising upon detecting a failure of a givenone of the digital information devices storing one of the firstoperational data tables, configuring the corresponding secondoperational data table as a new master cache, creating a new secondoperational data table, and configuring the new second operational datatable as a backup operational data table for the new master operationaldata table.
 8. The method according to claim 1, wherein the digitalinformation device comprises a module of a storage controller, andwherein the storage configuration comprises a redundant array ofindependent disks (RAID) configuration.
 9. The method according to claim1, wherein the metadata for the logical volume is selected from a listcomprising a partition table, a transaction distribution table and adisk distribution table.
 10. An apparatus, comprising: multiple storagedevices and multiple digital information devices arranged on a networkand having respective memories; and a separate processor coupled to eachof the respective memories and configured to divide data of a logicalvolume into data subsets, to define, for the logical volume, at leastone storage configuration for the data subsets distributed among therespective storage devices, to write metadata for the logical volume toa first set of first metadata tables, each of the first metadata tablesconfigured as a master metadata table, to divide the first set of firstmetadata tables into metadata subsets having a one-to-one correspondencewith the data subsets, to distributedly store the metadata subsets amongthe multiple digital information devices, the storage configuration ofthe data subsets independent from the storing of the metadata subsets,to copy the metadata from the first set of first metadata tables to asecond set of corresponding second metadata tables in a one-to-onecorrespondence with the first metadata tables, each of the secondmetadata tables configured as a backup metadata table, and todistributedly store the second metadata tables among the multipledigital information devices so that a given first metadata table and thecorresponding second metadata table are stored to a different one of themultiple digital information devices, and so that the correspondingsecond metadata tables for the first metadata tables on a first of thedigital information devices are distributed among at least an additionaltwo of the digital information devices, and
 11. The apparatus accordingto claim 10, wherein upon modifying the metadata in a one of the firstmetadata tables while processing a storage request, a given one of theseparate processors is configured to update the corresponding secondmetadata table with the updated metadata.
 12. The apparatus according toclaim 10, wherein the separate processor is configured to distributedlystore the second metadata tables so that the corresponding secondmetadata tables for the first metadata tables on the first of thedigital information devices are distributed among at least an additionalthree of the digital information devices.
 13. The apparatus according toclaim 10, wherein upon detecting a failure of a given one of the digitalinformation devices storing one of the first metadata tables, theseparate processor is arranged to configure the corresponding secondmetadata table as a new master metadata table, to create a new secondmetadata table, and to configure the new second metadata table as abackup metadata table for the new master metadata table.
 14. Theapparatus according to claim 10, wherein the separate processor isconfigured to write operational data for the logical volume to a firstset of first operational data tables having a one-to-one correspondencewith the data subsets, each of the first operational data tablesconfigured as a master operational data table, the storage configurationof the data subsets independent from the storing of the cache subsets,to divide the first set of first operational data tables intooperational data subsets, to distributedly store the operational datasubsets among the multiple digital information devices, to copy theoperational data to a second set of corresponding second operationaldata tables in a one-to-one correspondence with the first operationaldata tables, each of the second operational data tables configured as abackup operational data table, to distributedly store the first and thesecond operational data tables among the multiple digital informationdevices so that a given first operational data table and thecorresponding second operational data table are stored to a differentone of the multiple digital information devices, and so that thecorresponding second operational data table for the first metadatatables on a first of the digital information devices are distributedamong at least an additional two of the digital information devices, andwherein upon modifying the operational data in a one of the firstoperational data tables while processing a storage request, the separateprocessor is configured to update the corresponding operational datatable with the updated operational data.
 15. The apparatus according toclaim 14, wherein the separate processor is configured to distributedlystore the second operational data tables so that the correspondingsecond operational data tables for the first operational data tables onthe first of the digital information are distributed among at least anadditional three of the digital information devices.
 16. The apparatusaccording to claim 14, wherein upon detecting a failure of a given oneof the digital information devices storing one of the first operationaldata tables, the separate processor is arranged to configure thecorresponding second operational data table as a new master operationaldata table, to create a new second operational data table, and toconfigure the new second operational data tables as a backup operationaldata table for the new master cache.
 17. A computer program product, thecomputer program product comprising: a non-transitory computer readablestorage medium having computer readable program code embodied therewith,the computer readable program code comprising: computer readable programcode configured to arrange multiple storage devices and multiple digitalinformation devices having respective memories to communicate within anetwork; computer readable program code configured to divide data of alogical volume into data subsets; computer readable program codeconfigured to define, for the logical volume, at least one storageconfiguration for the data subsets distributed among the respectivestorage devices; computer readable program code configured to writemetadata for the logical volume to a first set of first metadata tables,each of the first metadata tables configured as a master metadata table;computer readable program code configured to divide the first set offirst metadata tables into metadata subsets having a one-to-onecorrespondence with the data subsets; computer readable program codeconfigured to distributedly store the metadata subsets among themultiple digital information devices , the storage configuration of thedata subsets independent from the storing of the metadata subsets;computer readable program code configured to copy the metadata from thefirst set of first metadata tables to a second set of correspondingsecond metadata tables in a one-to-one correspondence with the firstmetadata tables, each of the second metadata tables configured as abackup metadata table; computer readable program code configured todistributedly store the second metadata tables among the multipledigital information devices so that a given first metadata table and thecorresponding second metadata table are stored to a different one of themultiple digital information devices, and so that the correspondingsecond metadata tables for the first metadata tables on a first of thedigital information devices are distributed among at least an additionaltwo of the digital information devices; and
 18. The computer programproduct according to claim 17, and comprising computer readable programcode configured to update, upon modifying the metadata in a one of thefirst metadata tables while processing a storage request, thecorresponding second metadata table with the updated metadata.
 19. Thecomputer program product according to claim 17, and comprising computerreadable program code configured to distributedly store the secondmetadata tables so that the corresponding second metadata tables for thefirst metadata tables on the first of the digital information devicesare distributed among at least an additional three of the digitalinformation devices.
 20. The computer program product according to claim17, and comprising upon detecting a failure of a given one of thedigital information devices storing one of the first metadata tables,computer readable program code configured to configure the correspondingsecond metadata table as a new master metadata table, to create a newsecond metadata table, and to configure the new second metadata table asa backup metadata table for the new master metadata table.
 21. Thecomputer program product according to claim 17, and comprising computerreadable program code configured to write operational data for thelogical volume to a first set of first operational data tables having aone-to-one correspondence with the data subsets, each of the firstoperational data tables configured as a master operational data table,to divide the first set of first operational data tables intooperational data subsets, the storage configuration of the data subsetsindependent from the storing of the operational data subsets, todistributedly store the operational data subsets among the multipledigital information devices, to copy the operational data to a secondset of corresponding second operational data tables in a one-to-onecorrespondence with the first operational data tables, each of thesecond operational data tables configured as a backup operational datatable, to distributedly store the first and the second operational datatables among the multiple digital information devices so that a givenfirst operational data table and the corresponding second operationaldata table are stored to a different one of the multiple digitalinformation devices, and so that the corresponding second operationaldata tables for the first metadata tables on a first of the digitalinformation devices are distributed among at least an additional two ofthe digital information devices, and upon modifying the operational datain a one of the first operational data tables while processing a storagerequest, computer readable program code configured to update thecorresponding operational data table with the updated operational data.22. The computer program product according to claim 21, and comprisingcomputer readable program code configured to distributedly store thesecond operational data tables so that the corresponding secondoperational data tables for the first operational data tables on thefirst of the digital information are distributed among at least anadditional three of the digital information devices.
 23. The computerprogram product according to claim 21, and comprising upon detecting afailure of a given one of the digital information devices storing one ofthe first operational data tables, computer readable program codeconfigured to configure the corresponding second operational data tableas a new master cache, to create a new second operational data table,and to configure the new second operational data table as a backupoperational data table for the new master operational data table.